/*
 * $Header: /data/cvs/alin/nanhill-platform/src/main/java/org/nanhill/commons/dbutil/QueryLoader.java,v 1.1 2006/09/20 06:55:57 alin Exp $
 * $Revision: 1.1 $
 * $Date: 2006/09/20 06:55:57 $
 * 
 * ====================================================================
 *
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2003 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution, if
 *    any, must include the following acknowledgement:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgement may appear in the software itself,
 *    if and wherever such third-party acknowledgements normally appear.
 *
 * 4. The names "The Jakarta Project", "Commons", and "Apache Software
 *    Foundation" must not be used to endorse or promote products derived
 *    from this software without prior written permission. For written
 *    permission, please contact apache@apache.org.
 *
 * 5. Products derived from this software may not be called "Apache"
 *    nor may "Apache" appear in their names without prior written
 *    permission of the Apache Software Foundation.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 */

package org.nanhill.commons.dbutil;

import java.io.IOException;
import java.io.InputStream;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

/**
 * <code>QueryLoader</code> loads properties files filled with query name to
 * SQL mappings. It acts as a registry for sets of queries so that multiple
 * copies of the same queries aren't loaded into memory. This class is thread
 * safe.
 * 
 * @author David Graham
 */
public class QueryLoader {

	/**
	 * The Singleton instance of this class.
	 */
	private static final QueryLoader instance = new QueryLoader();

	/**
	 * Return an instance of this class.
	 * 
	 * @return The Singleton instance.
	 */
	public static QueryLoader instance() {
		return instance;
	}

	/**
	 * Maps query set names to Maps of their queries.
	 */
	private Map<String, Map<String, String>> queries = new HashMap<String, Map<String, String>>();

	/**
	 * QueryLoader constructor.
	 */
	protected QueryLoader() {
		super();
	}

	/**
	 * Loads a Map of query names to SQL values. The Maps are cached so a
	 * subsequent request to load queries from the same path will return the
	 * cached Map.
	 * 
	 * @param path
	 *            The path that the ClassLoader will use to find the file. This
	 *            is <strong>not</strong> a file system path. If you had a
	 *            jarred Queries.properties file in the com.yourcorp.app.jdbc
	 *            package you would pass
	 *            "/com/yourcorp/app/jdbc/Queries.properties" to this method.
	 * @throws IOException
	 * @throws IllegalArgumentException
	 *             if the ClassLoader can't find a file at the given path.
	 */
	public synchronized Map<String, String> load(String path) throws IOException {

		Map<String, String> queryMap = (Map<String, String>) this.queries
				.get(path);

		if (queryMap == null) {
			queryMap = this.loadQueries(path);
			this.queries.put(path, queryMap);
		}

		return queryMap;
	}

	/**
	 * Loads a properties file into a Map object.
	 * 
	 * @param path
	 *            The path that the ClassLoader will use to find the file.
	 * @throws IOException
	 */
	@SuppressWarnings("unchecked")
	private Map<String, String> loadQueries(String path) throws IOException {
		InputStream in = QueryLoader.class.getResourceAsStream(path);

		if (in == null) {
			throw new IllegalArgumentException(path + " not found.");
		}

		Properties props = new Properties();
		props.load(in);

		// Copy to HashMap for better performance
		return new HashMap(props);
	}

	/**
	 * Removes the queries for the given path from the cache.
	 * 
	 * @param path
	 *            The path that the queries were loaded from.
	 */
	public synchronized void unload(String path) {
		this.queries.remove(path);
	}

}
